Fuel cell vehicle

ABSTRACT

A fuel cell vehicle is provided. At the time of regeneration of electric power by a motor, an ECU places a DC/DC converter in a direct connection state under control, and stores electric power in a battery while decreasing oxygen concentration or hydrogen concentration by a gas supply unit to decrease electric power generated by a fuel cell.

CROSS-REFERENCE TO RELATED APPLICATION

This application is based upon and claims the benefit of priority fromJapanese Patent Application No. 2011-196912 filed on Sep. 9, 2011, ofwhich the contents are incorporated herein by reference.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to a fuel cell vehicle which makes itpossible to prevent degradation of a fuel cell for performing powergeneration by electrochemical reactions of reactant gases(oxygen-containing gas and fuel gas), highly efficiently collectregenerative electric power generated by a drive motor, and improvepower generation efficiency of the fuel cell.

2. Description of the Related Art

A fuel cell employs a membrane electrode assembly (MEA) which includesan anode, a cathode, and a solid polymer electrolyte membrane interposedbetween the anode and the cathode. The solid polymer electrolytemembrane is formed by impregnating a thin membrane of perfluorosulfonicacid with water, for example. Each of the cathode and the anode has agas diffusion layer such as a carbon paper, and an electrode catalystlayer of catalyst particles of platinum alloy or the like (hereinafteralso referred to as the Pt catalyst) supported on porous carbonparticles. The carbon particles are deposited uniformly on the surfaceof the gas diffusion layer. The electrode catalyst layer of the anodeand the electrode catalyst layer of the cathode are fixed to bothsurfaces of the solid polymer electrolyte membrane, respectively.

A technique of suppressing degradation of the fuel cell is proposed inJapanese Laid-Open Patent Publication No. 2007-005038 (hereinafterreferred to as JP 2007-005038 A). In the technique proposed in JP2007-005038 A, power generation of the fuel cell is performed such thatan oxidation reduction electric potential where sintering phenomenon ofthe Pt catalyst (aggregation of the Pt catalyst) occurs is avoided.

SUMMARY OF THE INVENTION

In a fuel cell vehicle, regenerative electric power is generated at thetime of deceleration or the like of the vehicle. Preferably, theregenerative electric power should be supplied to a battery for chargingin order to improve the system efficiency.

According to the disclosure of JP 2007-005038 A, even if the openingdegree of an accelerator pedal is increased, electric power is suppliedfrom the battery with the output voltage of the fuel cell being limitedto about 0.7 V until the SOC value of the battery becomes less than afirst charging threshold (lower limit target value of the SOC value).When it is detected that the SOC value becomes less than the firstcharging threshold, by increasing electric power generated in powergeneration of the fuel cell, the output voltage is decreased from about0.7 V to charge the battery. Thereafter, even if the opening angle ofthe accelerator pedal is decreased, the battery is charged whilemaintaining the state where electric power generated in power generationof the fuel cell is increased until the SOC value exceeds a secondcharging threshold (upper limit target value of the SOC value).

In this manner, by limiting the output voltage of the fuel cell to anelectric potential equal to or less than the oxidation reductionpotential, it is possible to suppress degradation of the fuel cell.However, even if the accelerator opening degree is decreased, i.e.,during the state where regenerative energy can be collected, the statewhere electrical energy generated by the fuel cell is increased iscontinued, and as a result, the system efficiency becomes poordisadvantageously.

According to the disclosure of Japanese Laid-Open Patent Publication No2006-073506 (hereinafter referred to as JP 2006-073506 A), a fuel cellvehicle is configured to drive a vehicle drive motor using electricpower of a fuel cell and electric power of an energy storage device atthe voltage boosted up to the voltage of the fuel cell by a DC/DCconverter.

In the fuel cell vehicle disclosed in JP 2006-073506 A, a contactor forestablishing electrical connection or disconnecting electricalconnection between the fuel cell and the vehicle drive motor is used. Atthe time of collecting regenerative electric power, the contactor isopened for electrically disconnecting the fuel cell from the vehicledrive motor, and the DC/DC converter is placed in a direct connectionstate (non-switching state) under control, and the regenerative electricpower charges (i.e., is collected in) the energy storage device throughthe DC/DC converter in the direct connection state. Therefore, at thetime of regenerating electric power, the switching loss of the DC/DCconverter becomes substantially zero, and the regenerative electricpower can be collected highly efficiently.

However, it is not preferable to open the contactor during powergeneration of the fuel cell, in terms of durability of the contactor andin terms of noises generated when the contactor is opened.

The present invention has been made in consideration of these problems,and an object of the preset invention is to provide a fuel cell vehiclewhich makes it possible to collect regenerative electric power highlyefficiently without opening a contactor during power generation of afuel cell.

A fuel cell vehicle according to the present invention includes a fuelcell for performing power generation by inducing, by catalyst, reactionof a first gas containing oxygen and a second gas containing hydrogensupplied to the fuel cell, a gas supply unit for supplying at least oneof the first gas and the second gas to the fuel cell, a voltageregulator for regulating output voltage of the fuel cell, a drive motoras a load driven by electric power outputted from the fuel cell, anenergy storage device for storing electric power regenerated by thedrive motor, and a control unit for controlling the fuel cell, the gassupply unit, the voltage regulator, the drive motor, and the energystorage device. At the time of regeneration of electric power by thedrive motor, the control unit places the voltage regulator in a directconnection state under control, and store electric power in the energystorage device while decreasing oxygen concentration or hydrogenconcentration by the gas supply unit to decrease electric poweroutputted from the fuel cell.

In the present invention, at the time of regeneration of electric powerby the drive motor, the voltage regulator is placed in a directconnection state under control, and electric power is stored in theenergy storage device while decreasing the oxygen concentration orhydrogen concentration by the gas supply unit to decrease the electricpower outputted from the fuel cell. In this manner, unlike the case ofJapanese Laid-Open Patent Publication No. 2006-073506, it is notnecessary to open the contactor during power generation of the fuelcell, and regenerative electric power can be collected highlyefficiently.

In this case, at the time of decreasing electric power outputted fromthe fuel cell, the control unit decreases the electric power outputtedfrom the fuel cell to a high efficiency power generation range of thefuel cell. In this manner, regenerative electric power can be collectedfurther efficiently.

At the time of regeneration of electric power by the drive motor, if thevoltage of the energy storage device is equal to or smaller than a lowerlimit voltage value of an oxidation reduction progress voltage range ofthe fuel cell where oxidation reduction proceeds, the control unitplaces the voltage regulator in the direct connection state undercontrol, and stores electric power in the energy storage device undercontrol while decreasing the oxygen concentration or hydrogenconcentration by the gas supply unit to decrease electric poweroutputted from the fuel cell. In this manner, degradation of the fuelcell is prevented, and the regenerative electric power can be collectedhighly efficiently.

Further, at the time of placing the voltage regulator in the directconnection state during regeneration of electric power by the drivemotor, the control unit determines whether or not regeneration torque ofthe drive motor exceeds a threshold value torque, and if the controlunit determines that the regeneration torque does not exceed thethreshold value torque, the control unit places the voltage regulator inthe direct connection state under control, and stores electric power inthe energy storage device under control while decreasing the oxygenconcentration or hydrogen concentration by the gas supply unit todecrease electric power outputted from the fuel cell. In this manner, inthe case where the regeneration torque is relatively small, theregenerative electric power can be collected into the energy storagedevice highly efficiently and reliably.

Further, if the control unit determines that the regeneration torqueexceeds the threshold value torque, the control unit does not place thevoltage regulator in the direct connection state under control andregulates, by the voltage regulator, the voltage of the fuel cell to avalue which is equal to or greater than the upper limit voltage of anoxidation reduction progress voltage range of the fuel cell whereoxidation reduction proceeds under control, and further the control unitstores electric power in the energy storage device while decreasing theoxygen concentration or hydrogen concentration by the gas supply unit todecrease electric power outputted from the fuel cell under control. Inthis manner, in the case where the regeneration torque is relativelylarge, the large regenerative electric power resulting from theregeneration torque can be collected into the energy storage devicehighly efficiently without degrading the fuel cell.

In the present invention, the regenerative electric power generatedduring regeneration by the drive motor driven by the energy storagedevice and the fuel cell can be collected into the energy storage devicehighly efficiently without opening the contactor between the fuel celland the drive motor during power generation of the fuel cell.

The above and other objects, features and advantages of the presentinvention will become more apparent from the following description whentaken in conjunction with the accompanying drawings in which preferredembodiments of the present invention are shown by way of illustrativeexample.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a diagram schematically showing an overall structure of a fuelcell vehicle equipped with a fuel cell system according to an embodimentof the present invention;

FIG. 2 is a block diagram showing a power system of the fuel cellvehicle;

FIG. 3 is a diagram schematically showing a structure of a fuel cellunit according to the embodiment;

FIG. 4 is a circuit diagram showing details of a DC/DC converteraccording to the embodiment;

FIG. 5 is a flow chart showing basic control (main routine) in anelectronic control unit (ECU);

FIG. 6 is a flow chart of calculating a system load;

FIG. 7 is a graph showing the relationship between the current rotationnumber of a motor and the estimated electric power consumed by themotor;

FIG. 8 is a graph showing an example of the relationship between thevoltage of a fuel cell of a fuel cell stack and degradation of the fuelcell;

FIG. 9 is a cyclic voltammetry diagram showing an example of theprogress of oxidation and the progress of reduction in the cases ofdifferent varying speeds in the voltage of the fuel cell;

FIG. 10 is a graph showing a normal current-voltage characteristic of afuel cell;

FIG. 11 is a graph showing the relationship between the cathodestoichiometric ratio and the cell current;

FIG. 12 is a flow chart illustrating a basic control mode according toenergy management and power generation control of a fuel cell;

FIG. 13 is a graph showing a plurality of power supply modes (e.g.,basic control mode) in the fuel cell;

FIG. 14 is a graph showing the relationship between the SOC value of abattery and the charging/discharging coefficient;

FIG. 15 is a graph showing the relationship between the target FCcurrent and the target oxygen concentration;

FIG. 16 is a graph showing the relationship between the target FCcurrent, and the target air pump rotation number and the target waterpump rotation number;

FIG. 17 is a graph showing the relationship between the target FCcurrent, and the target opening degree of a back pressure valve;

FIG. 18 is a flow chart showing torque control of the motor;

FIG. 19 is a flow chart illustrating operation of the embodiment;

FIG. 20 is a graph showing a power supply mode of the embodiment;

FIG. 21 is a graph showing the relationship between the power generationelectric power and the net efficiency of the fuel cell;

FIG. 22 is an explanatory diagram of an electrical current path throughwhich a battery is charged with regenerative electric power and FCelectric power in a direction connection state;

FIG. 23 is a time chart illustrating operation of the embodiment;

FIG. 24 is a diagram schematically showing a fuel cell unit according toa modified example of the embodiment;

FIG. 25 is a graph showing the relationship between the valve openingdegree of a circulation valve and the oxygen concentration in a cathodechannel;

FIG. 26 is a block diagram schematically showing a structure of anothermodified example of the fuel cell system;

FIG. 27 is a block diagram schematically showing a structure of stillanother modified example of the fuel cell system; and

FIG. 28 is a block diagram schematically showing a structure of yetanother modified example of the fuel cell system.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

FIG. 1 is a diagram schematically showing the overall structure of afuel cell vehicle 10 (hereinafter referred to as the “FC vehicle 10”)equipped with a fuel cell system 12 (hereinafter referred to as the “FCsystem 12”) according to an embodiment of the present invention. FIG. 2is a block diagram showing a power system of the FC vehicle 10. As shownin FIGS. 1 and 2, the FC vehicle 10 includes a traction motor 14 (drivemotor) and an inverter (bidirectional DC-AC converter) 16 in addition tothe FC system 12.

The FC system 12 includes a fuel cell unit 18 (hereinafter referred toas the “FC unit 18”), a high voltage battery (hereinafter referred to asthe “battery 20”) (energy storage device), a DC/DC converter (voltageregulator) 22, and an electronic control unit 24 (hereinafter referredto as the “ECU 24”) (control unit).

The motor 14 generates a driving force based on the electric powersupplied from the FC unit 18 and the battery 20, and rotates wheels 28using the driving force through a transmission 26. Further, the motor 14outputs electric power generated by regeneration (regenerative electricpower Preg) [W] to the battery 20 or the like (see FIG. 2) through theinverter 16 (also referred to as PDU (Power Drive Unit)) and the DC/DCconverter 22.

The inverter 16 has three-phase full bridge structure, and carries outDC/AC conversion to convert direct current into alternating current inthree phases. The inverter 16 supplies the alternating current to themotor 14, and supplies the direct current after AC/DC conversion as aresult of regeneration of the motor 14 to the battery 20 or the likethrough a DC/DC converter 22.

It should be noted that the motor 14 and the inverter 16 arecollectively referred to as a load 30 (also referred to as a main load30 in a case where it is necessary to distinguish between the load 30and loads of auxiliary devices (auxiliary device loads) 31 to bedescribed later). The main load 30 and the auxiliary device loads 31will be collectively referred to as a load 33 (also referred to as atotal load 33).

FIG. 3 is a diagram schematically showing a structure of the FC unit 18.The FC unit 18 includes a fuel cell stack 40 (hereinafter referred to asthe “FC stack 40” or the “FC 40”), an anode system 54 for supplyinghydrogen (fuel gas) to, and discharging the hydrogen (fuel gas) fromanodes of the FC stack 40, a cathode system 56 for supplying the air(oxygen-containing gas) to, and discharging the air (oxygen-containinggas) from cathodes of the FC stack 40, a cooling system 58 forcirculating coolant water (coolant) to cool the FC stack 40, and a cellvoltage monitor 42. For example, the FC stack 40 is formed by stackingfuel cells (hereinafter referred to as the “FC cells”) each including ananode, a cathode, and a solid polymer electrolyte membrane interposedbetween the anode and the cathode.

The anode system 54 includes a hydrogen tank 44 (gas supply unit), aregulator (second gas supply unit) 46, an ejector 48, and a purge valve50. The hydrogen tank 44 contains hydrogen as the fuel gas. The hydrogentank 44 is connected to the inlet of an anode channel 52 of FC 40through a pipe 44 a, a regulator 46, a pipe 46 a, an ejector 48, and apipe 48 a. Thus, the hydrogen in the hydrogen tank 44 can be supplied tothe anode channel 52 through the pipe 44 a or the like. A shut-off valve(not shown) is provided in the pipe 44 a. At the time of powergeneration of the FC stack 40, the shut-off valve is opened by the ECU24.

The regulator 46 regulates the pressure of the supplied hydrogen to apredetermined value, and discharges the hydrogen. That is, the regulator46 regulates the pressure on the downstream side (pressure of thehydrogen on the anode side) in response to the pressure (pilot pressure)of the air on the cathode side supplied through a pipe 46 b. Therefore,the pressure of the hydrogen on the anode side is linked to the pressureof the air on the cathode side. As described later, by changing therotation number or the like of an air pump (first gas supply unit) 60 soas to change the oxygen concentration, the pressure of the hydrogen onthe anode side changes as well.

The ejector 48 generates a negative pressure by ejecting hydrogen fromthe hydrogen tank 44 through a nozzle. By this negative pressure, theanode off gas can be sucked from a pipe 48 b.

The outlet of the anode channel 52 is connected to a suction port of theejector 48 through the pipe 48 b. The anode off gas discharged from theanode channel 52 flows through the pipe 48 b and again into the ejector48 to allow circulation of the anode off gas (hydrogen).

The anode off gas contains hydrogen that has not been consumed in theelectrode reaction at the anodes, and water vapor. Further, a gas-liquidseparator (not shown) is provided at the pipe 48 b forseparating/recovering water components (condensed water (liquid) andwater vapor (gas)) in the anode off gas.

Part of the pipe 48 b is connected to a dilution device (not shown)provided in a pipe 64 c, through a pipe 50 a, a purge valve 50, and apipe 50 b. When it is determined that power generation of the FC stack40 is not performed stably, the purge valve 50 is opened for apredetermined period in accordance with an instruction from the ECU 24.In the dilution device, the hydrogen in the anode off gas from the purgevalve 50 is diluted by the cathode off gas and discharged to atmosphere.

The cathode system 56 includes the air pump 60 (gas supply unit), ahumidifier 62, and a back pressure valve 64.

The air pump 60 compresses the external air (air), and supplies thecompressed air to the cathode. A suction port of the air pump 60 isconnected to the outside (outside of the vehicle, outside air of thevehicle) through a pipe 60 a, and an ejection port of the air pump 60 isconnected to the inlet of a cathode channel 74 through a pipe 60 b, thehumidifier 62, and a pipe 62 a. When the air pump 60 is operated inaccordance with an instruction from the ECU 24, the air pump 60 sucksthe air outside the vehicle through the pipe 60 a, compresses the suckedair, and supplies the compressed air to the cathode channel 74 of FC 40through the pipe 60 b or the like under pressure.

The humidifier 62 has a plurality of hollow fiber membranes 62 e havingwater permeability. The humidifier 62 humidifies the air flowing towardthe cathode channel 74 through the hollow fiber membranes 62 e byexchanging water components between the air flowing toward the cathodechannel 74 and the highly humidified cathode off gas discharged from thecathode channel 74.

A pipe 62 b, the humidifier 62, a pipe 64 a, the back pressure valve 64,the pipe 64 b, and the pipe 64 c are provided at the outlet of thecathode channel 74. The cathode off gas (oxygen-containing off gas)discharged from the cathode channel 74 is discharged from the pipe 64 cto the outside of the vehicle (to atmosphere) through the pipe 62 b orthe like.

For example, the back pressure valve 64 is a butterfly valve, and theopening degree of the back pressure valve 64 is controlled by the ECU 24to regulate the pressure of the air in the cathode channel 74. Morespecifically, if the opening degree of the back pressure valve 64becomes small, the pressure of the air in the cathode channel 74 isincreased, and oxygen concentration per volume flow rate (volumeconcentration) becomes high. Conversely, if the opening degree of theback pressure valve 64 becomes large, the pressure of the air in thecathode channel 74 is decreased, and oxygen concentration per volumeflow rate (volume concentration) becomes low.

A temperature sensor 72 is attached to the pipe 64 a. The temperaturesensor 72 detects the temperature of the cathode off gas, and outputsthe detected temperature to the ECU 24.

The cooling system 58 includes a water pump 80 and a radiator (heatradiator) 82. The water pump 80 circulates the coolant water (coolant),and an ejection port of the water pump 80 is connected to a suction portof the water pump 80 through a pipe 80 a, a coolant channel 84 of the FCstack 40, a pipe 82 a, the radiator 82, and a pipe 82 b in the orderlisted. When the water pump 80 is operated in accordance with aninstruction from the ECU 24, the coolant water is circulated between thecoolant channel 84 and the radiator 82 to cool the FC stack 40.

The cell voltage monitor 42 is a measurement instrument for detectingthe cell voltage Vcell of each of unit cells of the FC stack 40. Thecell voltage monitor 42 includes a monitor body, and a wire harnessconnecting the monitor body with each of the unit cells. The monitorbody scans all of the unit cells at predetermined intervals to detectthe cell voltage Vcell of each cell, and calculates the average cellvoltage and the lowest cell voltage. Then, the monitor body outputs theaverage cell voltage and the lowest cell voltage to the ECU 24.

As shown in FIG. 2, electric power from the FC stack 40 (hereinafterreferred to as the “FC electric power Pfc”) is supplied to the inverter16 and the motor 14 (during power running), and to the high voltagebattery 20 through the DC/DC converter 22 (during charging). Further,the FC electric power Pfc is supplied to the air pump 60, the water pump80, the air conditioner 90, a step down converter 92 (step down DC/DCconverter), a low voltage battery 94, an accessory 96, and the ECU 24. Abackflow prevention diode 98 is disposed between the FC stack 40 and theinverter 16 and the DC/DC converter 22. Further, the power generationvoltage of the FC 40 (hereinafter referred to as the “FC voltage Vfc”)is detected by a voltage sensor 100 (FIG. 4), and the power generationcurrent of the FC 40 (hereinafter referred to as the FC current Ifc”) isdetected by a current sensor 102. The FC voltage Vfc and the FC currentIfc are outputted to the ECU 24.

The battery 20 is an energy storage device (energy storage) containing aplurality of battery cells. For example, a lithium-ion secondarybattery, a nickel hydrogen secondary battery, or a capacitor can be usedas the battery 20. In the embodiment, the lithium-ion secondary batteryis used. The output voltage [V] of the battery 20 (hereinafter referredto as the “battery voltage Vbat or primary voltage V1”) is detected by avoltage sensor 120, and the output current [A] of the battery 20(hereinafter referred to as the “battery current Ibat or primary currentI1”) is detected by a current sensor 124. The battery voltage Vbat andthe battery current Ibat are outputted to the ECU 24. The remainingbattery level (state of charge) (hereinafter referred to as the “SOC”)[%] of the battery 20 is detected by a SOC sensor 104 (FIG. 2), andoutputted to the ECU 24.

Under the control of the ECU 24, the DC/DC converter 22 controls targetsto which the FC electric power Pfc from the FC unit 18, the electricpower [W] supplied from the battery 20 (hereinafter referred to as the“battery electric power Pbat”), and the regenerative electric power Pregfrom the motor 14 are supplied.

FIG. 4 shows an example of the DC/DC converter 22 in the presentembodiment. As shown in FIG. 4, one side of the DC/DC converter 22 isconnected to the primary side 1S where the battery 20 is provided, andthe other side of the DC/DC converter 22 is connected to the secondaryside 2S, which is connection points between the load 33 and the FC stack40.

The DC/DC converter 22 is basically a chopper type step up/down voltageconverter for increasing the voltage on the primary side 1S (primaryvoltage V1=Vbat) [V] to the voltage on the secondary side 2S (secondaryvoltage V2) [V] (V1≦V2), and decreasing the secondary voltage V2 to theprimary voltage V1 (V1=Vbat).

As shown in FIG. 4, the DC/DC converter 22 includes a phase arm UAinterposed between the primary side 1S and the secondary side 2S, and areactor 110.

The phase arm UA includes an upper arm element (an upper arm switchingelement 112 and a diode 114) as a high-side arm and a lower arm element(a lower arm switching element 116 and a diode 118) as a low-side arm.For example, MOSFET, IGBT, or the like is adopted in each of the upperarm switching element 112 and the lower arm switching element 116.

The reactor 110 is interposed between the middle point (commonconnection point) of the phase arm UA and the positive electrode of thebattery 20. The reactor 110 is operated to release and accumulate energyduring voltage conversion between the primary voltage Vi and thesecondary voltage V2 by the DC/DC converter 22.

The upper arm switching element 112 is turned on when high level of agate drive signal (drive voltage) UH is outputted from the ECU 24, andthe lower arm switching element 116 is turned on when high level of agate drive signal (drive voltage) UL is outputted from the ECU 24.

The ECU 24 detects primary voltage V1 [V] by the voltage sensor 120provided in parallel with a smoothing capacitor 122 on the primary side1S, and detects electrical current on the primary side 1S (primarycurrent I1) [A] by the current sensor 124. Further, the ECU 24 detectssecondary voltage V2 [V] by a voltage sensor 126 provided in parallelwith a smoothing capacitor 128 on the secondary side 2S, and detectselectrical current on the secondary side 2S (secondary current I2) [A]by a current sensor 130.

At the time of stepping up the voltage of the DC/DC converter 22, at thefirst timing, the gate drive signal UL is switched to high level, andthe gate drive signal UH is switched to low level. Electricity from thebattery 20 is stored in the reactor 110 (through a current pathextending from the positive side of the battery 20 through the reactor110 and the lower arm switching element 116 to the negative side of thebattery 20). At the second timing, the gate drive signal UL is switchedto low level, and the gate drive signal UH is switched to low level.Electricity stored in the reactor 110 is supplied to the secondary side2S through the diode 114 (through a current path extending from thepositive side of the battery 20 through the reactor 110, the diode 114,the positive side of the secondary side 2S, the load 33 or the like, andthe negative side of the secondary side 2S to the negative side of thebattery 20). Thereafter, the first timing and the second timing, asmentioned above, are repeated during the period in which the voltage isstepped up.

At the time of stepping down the voltage of the DC/DC converter 22, atthe first timing, the gate drive signal UH is switched to high level,and the gate drive signal UL is switched to low level. Electricity fromthe secondary side 2S (FC stack 40 and/or the load 33 during electricpower regeneration by the motor 14) is stored in the reactor 110, andwith the electricity, the battery 20 is charged. At the second timing,the gate drive signal UH is switched to low level, and the gate drivesignal UL is switched to low level. Electricity stored in the reactor110 is supplied to the battery 20 through the diode 118, and with theelectricity, the battery 20 is charged. As can be seen from FIG. 2, theregenerative electric power Preg also can be supplied to loads 31 ofauxiliary devices such as the air pump 60. Thereafter, the first timingand the second timing are repeated during the period in which thevoltage is stepped down.

As described above, the DC/DC converter 22 is operated as a chopper typeconverter. Further, the DC/DC converter 22 can be operated as a directconnection type converter. In the case where the DC/DC converter 22 isoperated as the direction connection type converter, when the gate drivesignal UH is switched to high level at the duty of 100 [%], and the gatedrive signal UL is switched to low level at the duty of 0 [%], therebydischarging electric power from the battery 20, electrical current issupplied from the primary side 1S to the secondary side 2S through thediode 114 (e.g., electric power is supplied from the battery 20 to theload 33). At the time of charging the battery 20 with electric power,the electric power is supplied from the secondary side 2S to the battery20 through the upper arm switching element 112 (e.g., regenerativeelectric power Preg is supplied from the motor 14 to the battery 20).

The ECU 24 controls the motor 14, the inverter 16, the FC unit 18, theauxiliary device loads 31, the battery 20, the DC/DC converter 22, etc.through a communication line 140 (see e.g., FIG. 1). For implementingthe control, programs stored in a memory (ROM) are executed, anddetection values obtained by various sensors such as the cell voltagemonitor 42, the flow rate sensors 68, the temperature sensor 72, thevoltage sensors 100, 120, 126, the current sensors 102, 124, 130, andthe SOC sensor 104 are used.

In addition to the above sensors, the various sensors herein include anopening degree sensor 150, a motor rotation number sensor 152, and avehicle velocity sensor 154 (FIG. 1). The opening degree sensor 150detects the opening degree (accelerator opening degree) θp [degrees] ofan accelerator pedal 156, which is an accelerator pedal angle, and themotor rotation number sensor 152 detects the rotation number Nm [rpm] ofthe motor 14. The vehicle velocity sensor 154 detects the vehiclevelocity Vs [km/h, kmph] of the FC vehicle 10. Further, a main switch158 (hereinafter referred to as the “main SW 158”) is connected to theECU 24. The main SW 158 switches between supply and non-supply of theelectric power from the FC unit 18 and the battery 20 to the motor 14.This main SW 158 can be operated by a user and corresponds to theignition switch of an engine vehicle.

The ECU 24 includes a microcomputer. Further, as necessary, the ECU 24has a timer and input/output (I/O) interfaces such as an A/D converterand a D/A converter. The ECU 24 may comprise only a single ECU.Alternatively, the ECU 24 may comprise a plurality of ECUs for each ofthe motor 14, the FC unit 18, the battery 20, and the DC/DC converter22.

After the load required by the FC system 12, i.e., required by the FCvehicle 10 as a whole is determined based on the state of the FC stack40, the state of the battery 20, and the state of the motor 14, and alsobased on inputs (load requests) from various switches and varioussensors, the ECU 24 determines allocation (shares) of loads throughadjustment, and more specifically determines a good balance among a loadwhich should be assigned to the FC stack 40, a load which should beassigned to the battery 20, and a load which should be assigned to theregenerative power supply (motor 14), and sends instructions to themotor 14, the inverter 16, the FC unit 18, the battery 20, and the DC/DCconverter 22.

As shown in FIG. 4, a positive electrode contactor CTR1 is provided at aposition denoted by a black point at the anode side of the diode 98 onthe positive electrode side of the FC stack 40, and a negative electrodecontactor CTR2 is provided at a position denoted by a black point on thenegative electrode side of the FC stack 40. The contactors CRT1, CRT2are turned on (closed) when the main SW 158 is turned on (closed) at thetime of starting operation (starting power generation) of the fuel cellvehicle 10, and the contactors CRT1, CRT2 are turned off (opened) whenthe main SW 158 is turned off (opened) at the time of stopping operation(stopping power generation) of the fuel cell vehicle 10. The main SW 158may be configured to have functions of the contactors CTR1, CTR2.

[Explanation of Basic Control Operation]

Next, operation of basic control in the ECU 24 will be described. On thepremise of the basic control, an embodiment will be described later.

FIG. 5 is a flow chart showing basic control (main routine) in the ECU24. In step S1, the ECU 24 determines whether or not the main SW 158 isin an ON state. If the main SW 158 is not in the ON state (S1: NO), stepS1 is repeated. If the main SW 158 is in the ON state (S1: YES), thecontrol proceeds to step S2. In step S2, the ECU 24 calculates the load(system load Psys or system requirement load Psys) [W] required by theFC system 12.

In step S3, the ECU 24 performs energy management of the FC system 12based on the calculated system load Psys. The energy management hereinis intended to suppress degradation of the FC stack 40, and improve theefficiency in the output (system efficiency) of the entire FC system 12.

In step S4, based on the results of energy management operation, the ECU24 implements control for peripheral devices of the FC stack 40, i.e.,the air pump 60, the back pressure valve 64, and the water pump 80 (FCpower generation control). In step S5, the ECU 24 implements torquecontrol of the motor 14.

In step S6, the ECU 24 determines whether or not the main SW 158 is inan OFF state. If the main SW 158 is not in the OFF state (S6: NO), thecontrol returns to step S2. If the main SW 158 is in the OFF state (S6:YES), the current process is finished.

FIG. 6 is a flow chart for calculating the system load Psys in step S2.In step S11, the ECU 24 reads the opening degree θp of the acceleratorpedal 156 from the opening degree sensor 150. In step S12, the ECU 24reads the rotation number Nm [rpm] of the motor 14 from the motorrotation number sensor 152.

In step S13, the ECU 24 calculates the estimated electric power Pm [W]consumed by the motor 14 based on the opening degree θp and the rotationnumber Nm. Specifically, in a map (characteristics) as shown in FIG. 7,the relationship between the rotation number Nm [rpm] of the motor 14and the estimated electric power Pm [W] consumed by the motor 14 isstored for each opening degree θp. For example, in the case where theopening degree θp is θp1, a characteristic 180 is used. Likewise, in thecases where the opening degrees θp are θp2, θp3, θp4, θp5, and θp6,characteristics 182, 184, 186, 188, and 190 are used, respectively.After the characteristic indicating the relationship between therotation number Nm and the estimated consumed electric power Pm isdetermined based on the opening degree θp, the estimated consumedelectric power Pm in correspondence with the rotation number Nm isdetermined based on the determined characteristic. During accelerationin the power running mode, the estimated consumed electric power Pm hasa positive value. During deceleration in the regenerating mode, theestimated consumed electric power Pm has a negative value. That is, inthis mode, the estimated consumed electric power Pm indicates estimatedregenerative electric power.

In step S14, the ECU 24 reads data of the current operating conditionsfrom the load 31 of auxiliary devices. For example, as shown in FIG. 2,the auxiliary devices herein include high voltage auxiliary devices,such as the air pump 60, the water pump 80, and the air conditioner 90,and low voltage auxiliary devices, such as the low voltage battery 94,the accessory 96, and the ECU 24. For example, as for the operatingcondition of the air pump 60, the rotation number Nap [rpm] of the airpump 60 is read. As for the operating condition of the water pump 80,the rotation number Nwp [rpm] of the water pump 80 is read. As for theoperating condition of the air conditioner 90, output settings of theair conditioner 90 are read.

In step S15, the ECU 24 calculates the electric power Pa [W] consumed bythe auxiliary devices depending on the present operating conditions ofthe auxiliary devices.

In step S16, the ECU 24 calculates the sum of the estimated electricpower Pm consumed by the motor 14 and electric power Pa consumed by theauxiliary devices (provisional system load Pm+Pa) to determine theestimated electric power consumption in the entire FC vehicle 10. Thatis, the ECU 24 calculates the system load Psys (Psys=Pm+Pa, also denotedas Psys→Pm+Pa).

As described above, the energy management according to the presentembodiment is aimed to suppress degradation of the FC stack 40, andimprove the efficiency in the output of the entire FC system 12.

FIG. 8 shows an example of the relationship between the voltage of theFC cell of the FC stack 40 (cell voltage Vcell) [V] and the degradationD of the cell. That is, a curve (characteristic) 142 in FIG. 8 shows therelationship between the cell voltage Vcell and the degradation D.

In FIG. 8, in a region below the voltage v1 (e.g., 0.5V), reductionreaction of platinum (oxidized platinum) in the FC cell proceedsseverely, and aggregation of platinum occurs excessively (hereinafterreferred to as the “platinum aggregation-increasing region R1” or the“aggregation-increasing region R1”). In a region from the voltage v1 tothe voltage v2 (e.g., 0.8 V), reduction reaction proceeds stably(hereinafter referred to as the “stable platinum reduction region R2”,or the “stable reduction region R2”, or the “region R2 of a voltagerange where catalyst reduction proceeds stably”).

In a region from the voltage v2 to the voltage v3 (e.g. 0.9 V),oxidation-reduction reaction of platinum proceeds (hereinafter referredto as the “platinum oxidation reduction progress region R3” or the“oxidation reduction progress region R3”). In a region from the voltagev3 to the voltage v4 (e.g., 0.95V), oxidation reaction of platinumproceeds stably (hereinafter referred to as the stable platinumoxidation region R4” or the “stable oxidation region R4”). In a regionfrom the voltage v4 to OCV (open circuit voltage), oxidation of carbonin the FC cell proceeds (hereinafter referred to as the “carbonoxidation progress region R5”).

As described above, in FIG. 8, if the cell voltage Vcell is in thestable platinum reduction region R2 or the stable platinum oxidationregion R4, degradation of the FC cell occurs to a smaller extent. Incontrast, if the cell voltage Vcell is in the platinum aggregationincreasing region R1, the platinum oxidation reduction progress regionR3, or the carbon oxidation progress region R5, degradation of the FCcell occurs to a greater extent.

In FIG. 8, on the face of it, the curve (characteristic) 142 is uniquelydetermined. However, in practice, the curve (characteristic) 142 variesdepending on variation of the cell voltage Vcell (varying speed Acell)[V/sec] per unit time.

FIG. 9 is a cyclic voltammetry diagram showing an example of theprogress of oxidation and the progress of reduction in the cases ofdifferent varying speeds Acell. In FIG. 9, a solid curve 170(characteristic) shows a case where the varying speed Acell is high, anda dotted curve 172 (characteristic) shows a case where the varying speedAcell is low. As can be seen from FIG. 9, since the degree of theprogress in oxidation or reduction varies depending on the varying speedAcell, the voltages v1 to v4 cannot necessarily be determined uniquely.Further, the voltages v1 to v4 may change depending on the individualdifference in the FC cell. Therefore, preferably, the voltages v1 to v4should be set such that errors are reflected in the theoretical values,the simulation values, or the measured values.

Further, in the current-voltage (I-V) characteristic of the FC cell, asin the case of normal fuel cells, as the cell voltage Vcell decreases,the cell current Icell [A] is increased (see a I-V characteristic 162indicated by “normal” in FIG. 10, and hereinafter referred to as “normalI-V characteristic”). Additionally, the power generation voltage (FCvoltage Vfc) of the FC stack 40 is obtained by multiplying the cellvoltage Vcell by the serial connection number Nfc. The serial connectionnumber Nfc indicates the number of FC cells connected in series in theFC stack 40. The serial connection number Nfc is also simply referred toas the “cell number”.

The normal I-V characteristic 162 in FIG. 10 is obtained when oxygen isin a rich state, i.e., the cathode stoichiometric ratio (which is nearlyequal to oxygen concentration) is the normal stoichiometric ratio ormore. Stated otherwise, when oxygen is in a rich state, the oxygenconcentration is the normal oxygen concentration or more. The cathodestoichiometric ratio herein means (the flow rate of the air supplied tothe cathode)/(the flow rate of the air consumed by power generation). Inthe present embodiment, the cathode stoichiometric ratio is also simplyreferred to as the stoichiometric ratio.

The expression “oxygen is in a rich state” means a state where, as shownin FIG. 11, even if the cathode stoichiometric ratio (which is nearlyequal to oxygen concentration) is increased, the cell current Icelloutputted from the unit cell is kept substantially at a constant level.In this state, oxygen is present in a region above the normalstoichiometric ratio, where oxygen is saturated.

The stoichiometric ratio of hydrogen should be understood in the samemanner. That is, the anode stoichiometric ratio (which is nearly equalto hydrogen concentration) is represented by (the flow rate of thehydrogen supplied to the anode)/(the flow rate of the hydrogen consumedby power generation).

Next, in the energy management and the FC power generation control insteps S3 and S4, basic control (basic energy management·power generationcontrol) will be described with reference to a flow chart in FIG. 12.

In step S21, the ECU 24 calculates the charging/discharging coefficientα, and multiplies the system load Psys calculated in step S16 by thecalculated charging/discharging coefficient α to calculate target FCelectric power (Pfctgt←Psys×α).

The charging/discharging coefficient α herein is calculated based on thecurrent SOC value inputted from the SOC sensor 104 and a characteristic(map) 163 in FIG. 14. For example, measured values, simulation values orthe like may be used as the characteristic 163 in FIG. 14, and arestored in the ECU 24 in advance. In the embodiment, target SOC (targetenergy storage amount) of the battery 20 is 50 M. However, the presentinvention is not limited in this respect.

In the embodiment, as shown in FIG. 14, in a region where the SOC valueis less than 50 [%] (when charging is required), thecharging/discharging coefficient α is set to a value greater than “1”.In this manner, power generation is performed excessively in the FCstack 40, and the excessive electric power is used for charging thebattery 20. In a region where SOC value is greater than 50 [%] (when thebattery 20 is in a sufficiently charged state), the charging/dischargingcoefficient α is set to a value less than “1”. In this manner, shortageof electric power occurs in power generation of the FC stack 40, andelectric power discharged from the battery 20 is utilized to compensatefor the shortage of electric power.

For ease of understanding, in the following description, it is assumedthat the charging/discharging coefficient α is 1 (Pfctgt=Psys).

In step S22, the ECU 24 determines whether or not the target powergeneration electric power Pfctgt calculated in step S21 is a thresholdelectric power Pthp or more (Pfctgt≧Pthp).

The threshold electric power Pthp herein means a fixed value obtained bymultiplying the “cell voltage which is considered to cause nodegradation of catalyst (0.8 V, switching voltage, predeterminedvoltage)”, “the number of unit cells of the FC stack 40 (cell numberNfc)”, and the “current value Icellp in the case where the cell voltageis 0.8 V in the normal I-V characteristic 162 of the FC stack 40 (seeFIG. 10)”. This threshold electric power Pthp can be calculated by thefollowing expression (1). In FIG. 10, it should be noted that the axisof the target electric power Pfctgt is not linear.

Pthp=0.8 [V]×Nfc×Icellp  (1)

In the case where the target power generation electric power Pfctgt isthe threshold electric power Pthp or more (S22: YES), in step S23,voltage variable/current variable control (mode A control) isimplemented to obtain the target FC electric power Pfctgt.

This mode A control is mainly used when the target FC electric powerPfctgt is relatively high. In the state where the target oxygenconcentration Cotgt is kept in a normal state (including the oxygen richstate), the target FC voltage Vfctgt is regulated by the DC/DC converter22 thereby to control the FC current Ifc.

That is, as shown in FIG. 13, in the mode A control implemented when thetarget FC electric power Pfctgt is the threshold electric power Pthp ormore, the normal I-V characteristic 162 of the FC stack 40 (same as thatshown in FIG. 10) is used. In the mode A control, the target FC currentIfctgt is calculated in correspondence with the target FC electric powerPfctgt. Further, the target FC voltage Vfctgt is calculated incorrespondence with the target FC current Ifctgt. Then, the ECU 24controls the DC/DC converter 22 such that the FC voltage Vfc isregulated to be the target FC voltage Vfctgt. That is, the FC voltageVfc is controlled to control the FC current Ifc by increasing (steppingup) the primary voltage V1 by the DC/DC converter 22 such that thesecond voltage V2 is regulated to be the target FC voltage Vfctgt.

In the mode A control as described above, even if the target FC electricpower Pfctgt is the threshold electric power Pthp or more, i.e., thesystem load Psys is high, the secondary voltage V2 (FC voltage Vfc) ischanged by the DC/DC converter 22 according to the normal I-Vcharacteristic 162 in correspondence with the target FC electric powerPfctgt, whereby basically the system load Psys can be covered by the FCelectric power Pfc.

In the determination in step S22, if the target FC electric power Pfctgtis less than the threshold electric power Pthp (step S22: NO), then instep S24, it is determined whether or not the target FC electric powerPfctgt calculated in step S21 is less than the threshold electric powerPthq (Pfctgt<Pthq). For example, the threshold electric power Pthqcorresponding to the cell voltage of 0.9[V] (Vcell=0.9[V]) isdetermined. Therefore, the threshold electric power Pthq is smaller thanthe threshold electric power Pthp (Pthq<Pthp, see FIG. 13).

In the case where the determination in step S24 is negative, i.e., inthe case where the target FC electric power Pfctgt is less than thethreshold electric power Pthp, and equal to or more than the thresholdelectric power Pthq (step S24: NO, Pthq≦Pfctgt<Pthp), in step S25,voltage fixed/stoichiometric ratio variable current variable control(mode B control) is implemented. It should be noted that the voltagefixed/stoichiometric ratio variable current variable control of the modeB is implemented in the same manner as in mode C control and mode Econtrol to be described later. In contrast with the voltagevariable/current variable control in the mode A control as describedabove, the mode B control is common to the mode C control and the mode Econtrol in respect of the voltage fixed/current variable control.Therefore, this control is also referred to as the CVVC (ConstantVoltage Variable Current) control. In the mode D control as describedlater, voltage variable/stoichiometric ratio variable current variablecontrol is implemented. In this case, since the control is implementedin the direct connection state, the FC voltage Vfc changes in accordancewith the battery voltage Vbat, and the change in the FC voltage Vfc isnot significantly large.

The mode B control is mainly used when the system load Psys isrelatively medium. In the state where the target cell voltage Vcelltgt(=target FC voltage Vfctgt/cell number Nfc) is fixed to the referencevoltage (in the present embodiment, voltage v2 (=0.8 V)) which is set tobe equal to or less than the voltage below the oxidation reductionprogress region R3, the target oxygen concentration is variable, andthus, the FC current Ifc is variable.

That is, as shown in FIG. 13, in the mode B control, in the rangebetween the threshold electric power Pthq and the threshold electricpower Pthp, the cell voltage Vcell is kept at a constant level(Vcell=v2). In this state, the target oxygen concentration Cotgt isdecreased thereby to decrease the oxygen concentration Co.

As shown in FIG. 11, as the cathode stoichiometric ratio (which isnearly equal to the oxygen concentration Co) decreases, the cell currentIcell (FC current Ifc) is accordingly decreased. Therefore, in the statewhere the cell voltage Vcell is kept at a constant level (Vcell=v2=0.8V), by increasing or decreasing the target oxygen concentration Cotgt,it becomes possible to control the cell current Icell (FC current Ifc)and the FC electric power Pfc. The shortage of the FC electric power Pfcis assisted by the battery 20.

In this case, the ECU 24 regulates the step up voltage ratio of theDC/DC converter 22 thereby to fix the target FC voltage Vfctgt at thereference voltage (in the present embodiment, the voltage v2 (=0.8 V))which is set to be equal to or less than the voltage below the oxidationreduction progress region R3, and calculates the target FC currentIfctgt in correspondence with the target FC electric power Pfctgt.Further, the ECU 24 calculates the target oxygen concentration Cotgt incorrespondence with the target FC current Ifctgt on the premise that thetarget FC voltage Vfctgt is at the reference voltage (see FIGS. 11 and15). FIG. 15 shows the relationship between the target FC current Ifctgtand the target oxygen concentration Cotgt when the FC voltage Vfc is atthe reference voltage v2.

At this time, depending on the target oxygen concentration Cotgt, theECU 24 calculates, and sends instruction values to the respectivecomponents. The instruction values herein include the rotation number ofthe air pump 60 (hereinafter referred to as the “air pump rotationnumber Nap” or the “rotation number Nap”), the rotation number of thewater pump 80 (hereinafter referred to as the “water pump rotationnumber Nwp” or the “rotation number Nwp”), and the opening degree of theback pressure valve 64 (hereinafter referred to as the “back pressurevalve opening degree θbp” or the “opening degree θbp”).

That is, as shown in FIGS. 16 and 17, the target air pump rotationnumber Naptgt, the target water pump rotation number Nwptgt, and thetarget back pressure valve opening degree θbptgt are determineddepending on the target oxygen concentration Cotgt.

In this manner, the mode B control in step S25 is implemented.

Then, in step S26, the ECU 24 determines whether power generation by theFC stack 40 is stably performed or not. In the determination, if thelowest cell voltage inputted from the cell voltage monitor 42 is lowerthan the voltage obtained by subtracting a predetermined voltage fromthe average cell voltage (lowest cell voltage<(average cellvoltage—predetermined voltage)), the ECU 24 determines that powergeneration of the FC stack 40 is not stable. For example, measuredvalues, simulation values or the like may be used as the predeterminedvoltage.

If power generation is stable (S26: YES), the current process isfinished. If power generation is not stable (S26: NO), then in step S27,the ECU 24 increases the target oxygen concentration Cotgt by one stage(closer to normal concentration). Specifically, at least one of thecontrol to increase the rotation number Nap of the air pump 60 and thecontrol to decrease the opening degree θbp of the back pressure valve 64is performed by one stage.

In step S28, the ECU 24 determines whether or not the target oxygenconcentration Cotgt is less than the target oxygen concentration of thenormal I-V characteristic (normal oxygen concentration Conml). If thetarget oxygen concentration Cotgt is less than the normal oxygenconcentration Conml (S28: YES), the process returns to step S26. If thetarget oxygen concentration Cotgt is not less than the normal oxygenconcentration Conml (S28: NO), in step S29, the ECU 24 stops operationof the FC unit 18. That is, the ECU 24 stops supply of hydrogen and airto the FC stack 40 thereby to stop power generation of the FC stack 40.Then, the ECU 24 turns on an alarming lamp (not shown) to notify theoperator that there is a failure in the FC stack 40. It should be notedthat the ECU 24 supplies electric power from the battery 20 to the motor14 for allowing the FC vehicle 10 to continue running.

In the determination in step S24 as described above, if the target FCelectric power Pfctgt is less than the threshold electric power Pthq(step S24: YES), mode C control is implemented in step S30. As shown inFIG. 13, the mode C control is mainly used when the target FC electricpower Pfctgt is relatively low. The target cell voltage Vcelltgt(=target FC voltage Vfctgt/cell number) is fixed to the voltage (in thepresent embodiment, the voltage v3 (=0.9 V)) outside the oxidationreduction progress region R3, and the FC current Ifc is variable. Theshortage of the FC electric power Pfc is assisted by the battery 20, andexcessive electric power of the FC electric power Pfc is used forcharging the battery 20.

In the mode C control, as shown in FIG. 13, the cell voltage Vcell isfixed to a constant level (Vcell=v3). In this state, the target oxygenconcentration Cotgt is decreased thereby to decrease the oxygenconcentration Co.

As shown in FIG. 11, as the cathode stoichiometric ratio (which isnearly equal to the oxygen concentration Co) decreases, the cell currentIcell (FC current Ifc) is decreased. Thus, by increasing or decreasingthe target oxygen concentration Cotgt while keeping the cell voltageVcell at a constant level (Vcell=v3=0.9 V), it becomes possible tocontrol the cell current Icell (=FC current Ifc) and the FC electricpower Pfc. The shortage of the FC electric power Pfc is assisted by thebattery 20. Therefore, in the mode C control, the process in the samemanner as the control process in the mode B control in step S25 asdescribed above, and the process related to power generation stabilityin steps S26 to S29 are performed.

In this manner, basic control according to energy management and FCpower generation control of steps S3 and S4 is implemented.

Next, FIG. 18 is a flow chart showing torque control of the motor 14related to the process of step S5. In step S41, the ECU 24 reads vehiclevelocity Vs from the vehicle velocity sensor 154. In step S42, the ECU24 reads the opening degree θp of the accelerator pedal 156 from theopening degree sensor 150.

In step S43, the ECU 24 calculates a provisional target torque Ttgt_p[N·m] of the motor 14 based on the vehicle velocity Vs and the openingdegree θp. Specifically, a map representative of the relationshipbetween the vehicle velocity Vs, the opening degree θp, and theprovisional target torque Ttgt_p is stored in a memory (not shown), andthe target provisional torque Ttgt_p is calculated based on the map, thevehicle velocity Vs, and the opening degree θp.

In step S44, the ECU 24 determines whether or not the motor 14 isregenerating electric power. If the motor 14 is not regeneratingelectric power, the ECU 24 calculates the limit output of the motor 14(motor limit output Pm_lim) [W]. The motor limit output Pm_lim is equalto the limit value of electric power (limit supply electric powerPs_lim) [W] which can be supplied from the FC system 12 to the motor 14.Specifically, the limit supply electric power Ps_lim and the motor limitoutput Pm_lim are calculated by subtracting electric power Pa consumedby auxiliary device loads 31 from the sum of the FC electric power Pfcfrom the FC stack 40 and the limit value (limit output Pbat_lim) ofelectric power which can be supplied from the battery 20(Pm_lim=Ps_lim→Pfc+Pbat_lim−Pa).

In step S45, the ECU 24 calculates the torque limit value Tlim [N·m] ofthe motor 14. Specifically, the torque limit value Tlim is calculated bydividing the motor limit output Pm_lim by the vehicle velocity Vs(Tlim←Pm_lim/Vs).

In step S44, if the ECU 24 determines that the motor 14 is regeneratingelectric power, the ECU 24 calculates limit supply regenerative electricpower Ps_reglim. The limit supply regenerative electric power Ps_reglimis calculated by subtracting electric power Pa consumed by the load 31of the auxiliary devices from the sum of a limit value of electric powerwith which the battery 20 can be charged (charging limit Pbat_chglim)and the FC electric power Pfc from the FC stack 40 (Ps_reglim=Pbat_chglim+Pfc−Pa). If the motor 14 is regenerating electric power, instep S45, the ECU 24 calculates the regeneration torque limit valueTreglim [N·m] of the motor 14. Specifically, the torque limit value Tlimis calculated by dividing the limit supply regenerative electric powerPs_reglim by the vehicle velocity Vs (Tlim←Ps_reglim/Vs).

In step S46, the ECU 24 calculates the target torque Ttgt [N·m].Specifically, the ECU 24 calculates the target torque Ttgt by setting alimitation of the torque limit value Tlim to the provisional targettorque Ttgt_p. For example, in the case where the provisional targettorque Ttgt_p is the torque limit value Tlim or less, (Ttgt_p s Tlim),the provisional target torque Ttgt_p is directly used as the targettorque Ttgt (Ttgt←Ttgt_p). In the case where the provisional targettorque Ttgt_p exceeds the torque limit value Tlim (Ttgt_p>Tlim), thetorque limit value Tlim is used as the target torque Ttgt (Ttgt←Tlim).The calculated target torque Ttgt is used to control the motor 14.

Next, a process of charging the battery 20 under the energymanagement/power generation control according to the embodiment based onthe premise of the above described basic control modes (mode A control,mode B control, and mode C control) will be described with reference toa flow chart in FIG. 19.

Embodiment

In step S61, the ECU 24 determines whether the motor 14 having anegative target torque Ttgt is generating regenerative electric power(also referred to as during regeneration of electric power or at thetime of regenerating electric power). If the motor 14 is notregenerating electric power (step S61: NO), in step S62, the abovedescribed basic control mode is implemented.

If the motor 14 is regenerating electric power (step S61: YES), in stepS63, it is determined whether or not the battery voltage Vbat is equalto or less than a predetermined voltage (threshold voltage) valuecalculated by multiplying the voltage v2, which is the lower limitvoltage of the oxidation reduction progress region R3, by the serialconnection number Nfc (Vbat≦v2×Nfc).

If Vbat>v2×Nfc (step S63: NO), i.e., the battery voltage Vbat exceedsthe threshold voltage value v2×Nfc, the battery 20 has been chargedsufficiently, and the SOC value is high. Therefore, the regenerativeelectric power Preg is consumed by the load 31 of auxiliary devices orthe like.

If the determination of step S63 is affirmative (step S63: YES), acharging process by the regenerative electric power Preg and the FCelectric power (electric power generated by power generation) Pfc isperformed from step S64.

In step S64, it is determined whether or not the absolute value of thenegative target torque Ttgt (the absolute value being also referred toas the regeneration torque Treg) exceeds the regeneration torquethreshold value Tregth. The regeneration torque Treg means a torquegenerated when electric power generated by the motor 14 (regenerativeelectric power) is supplied to components such as the battery 20 toapply regenerative braking to wheels 28 by the motor 14.

If the regeneration torque Treg exceeds the regeneration torquethreshold value Tregth, the regenerative electric power Preg is large.Thus, in order to increase the efficiency of collecting thisregenerative electric power Preg, in step S65, the secondary voltage V2of the DC/DC converter 22 is controlled in order to decrease the targetpower generation electric power Pfctgt of the FC stack 40 to or belowthe threshold electric power Pthq (see FIG. 13) corresponding toVcell=0.9 [V].

In this embodiment, in step S65, the ECU 24 implements the mode Econtrol (CVVC control) shown in FIG. 20.

That is, in the mode E control (CVVC control), the DC/DC converter 22fixes the cell voltage Vcell of the FC stack 40 to the voltage Vlmi2(e.g., Vlmi2=0.95 [V]) where degradation D in the stable oxidationregion R4 in FIG. 8 is the smallest, and power generation is performedat a stoichiometric ratio lower than the normal stoichiometric ratiosuch that the FC current Ifc, which is the electrical current outputtedfrom the FC stack 40, is reduced by the amount of regenerativeelectrical current. (Alternatively, mode C control for fixing the cellvoltage Vcell to v3 (Vcell=v3) may be implemented.)

In the mode E control, the target power generation voltage Vfctgt isfixed to 0.95V×Nfc, the target oxygen concentration Cotgt is variable,and thus, the FC current Ifc is made variable.

In this manner, since the stoichiometric ratio is decreased to reducethe charging current by the FC stack 40, the collecting efficiency ofcharging components such as the battery 20 with the regenerativeelectric power Preg is improved. Thus, it becomes possible to reduceenergy loss.

In the mode E control, the power generation is performed under a lowoxygen condition at variable stoichiometric ratio and fixed voltage inthe stable oxidation region R4 where the cell voltage Vcell is high. Ascan be seen from FIG. 8, in terms of the degradation D, preferably,Vcell=Vlmi2, and in terms of the efficiency, preferably, the cellvoltage Vcell has a higher voltage of v4 (Vcell=v4). Therefore, in themode E control where voltage is fixed, the stoichiometric ratio isvariable and the electric current is variable (CVVC control), the cellvoltage Vcell should be fixed to any voltage between v3 and v4(v3<Vcell≦v4).

In practice, in step S66, it is determined whether or not the SOC valueof the battery 20 is a target SOCth or more (the target SOCth is apredetermined value, e.g., SOCth =50 [%]). If the SOC value is less thanthe target SOCth (step S66: NO), in step S67, the target powergeneration electric power Pfctgt of the FC stack 40 is set to the powergeneration electric power (power generation amount) Pfcηg [kW] (see FIG.21) where the net efficiency Net [%] is at the peak (maximum value), forpower generation.

FIG. 21 shows characteristics 302, 304, and 306 of the net efficiencyNetη [%] relative to the power generation electric power Pfc. Assumingthat the supplied hydrogen energy is Herg, the power generation electricpower of the FC stack 40 is Pfc (Pfc =Ifc x Vfc), electric powerconsumed by the load 31 of auxiliary devices is Pa, and the switchingloss of the DC/DC converter 22 is Pswloss, and the charging loss of thebattery 20 is Pbloss, the efficiency Netη [%] in each of thecharacteristics 302, 304, and 306 can be calculated as follows: In thecharacteristic 302, Netη={100×(Pfc−Pa−Pswloss−Pbloss)/Herg}, in thecharacteristic 304, Netη={100×(Pfc−Pa−Pswloss)/Herg}, in thecharacteristic 306, Netη={100×(Pfc−Pa)/Herg}. In all of the cases, theefficiency Netη [%] has the maximum value (peak value) at the powergeneration electric power Pfcηq.

In the determination of step S66, if the SOC value of the battery 20 isthe target SOCth or more, in step S68, the target power generationelectric power Pfctgt of the FC stack 40 is set to the power generationelectric power Pfcrηp in a region below the power generation electricpower (power generation amount) Pfcηq [kW] where the net efficiency Netη[%] is at the peak (maximum value) in FIG. 21.

In contrast, in the determination of step S64, if the absolute value ofthe target torque Ttgt (hereinafter referred to as the regenerationtorque Treg) is the regeneration torque threshold value Tregth or less(step S64: NO), in order to efficiently collect the regenerativeelectric power, in step S69, the DC/DC converter 22 is placed in thedirect connection state under control.

In order to place the DC/DC converter 22 in the direct connection stateunder control, the gate drive signal UL is switched to the low level toturn off the lower arm switching element 116, and the gate drive signalUH is switched to the high level to turn on the upper arm switchingelement 112. At the same time, in step S69, the FC stack 40 is operatedunder the mode D control shown by a hatched area in FIG. 20. In the modeD control, the target power generation voltage Vfctgt is set to Vbat,i.e., in the direct connection state (Vfctgt←Vbat). The cell voltageVcell is regulated between v1 and v2 (v1≦Vcell>v2). The target oxygenconcentration Cotgt is variable, and thus, the FC current Ifc isvariable. The battery 20 is charged with the regenerative electric powerPreg and the FC electric power Pfc through the electrical current pathshown in FIG. 22.

Also in the direct connection state in step S69, as described withrespect to step S66, S67, and S68, in step S70, it is determined whetheror not the SOC value of the battery 20 is the target SOCth or more,e.g., 50 [%] or more. If the SOC value is less than the target SOCth(step S70: NO), in step S71, the target power generation electric powerPfctgt of the FC stack 40 is set to the power generation electric power(power generation amount) Pfcηg [kW] where the net efficiency Netη [%]is at the peak (maximum) (see FIG. 21), for power generation.

In the determination of step S70, if the SOC value of the battery 20 isthe target SOCth or more, in step S72, the target power generationelectric power Pfctgt of the FC stack 40 is set to the power generationelectric power Pfcθq below the power generation electric power (powergeneration amount) Pfcηq [kW] where the net efficiency Netη [%] is atthe peak (maximum), for power generation.

FIG. 23 is a time chart showing a process of step S69 in the case whereit is determined that the motor 14 is regenerating electric power attime t11.

At time t11, when deceleration of the vehicle velocity Vs is started,regeneration of electric power is started. Therefore, the air pumprotation number Nap is decreased in order to limit power generation ofthe FC stack 40. Thus, the FC voltage Vfc is increased, the FC currentIfc is decreased, and then the determination in step S63(Vbat<v2×Nfc=Vfc) becomes affirmative. At time t12, the directionconnection flag is placed in the ON state. As shown in FIG. 22, sincethe ON state of the upper arm switching element 112 (direct connectionstate of the DC/DC converter 22) continues, after time t12, the FCvoltage Vfc follows the battery voltage Vbat. During regeneration ofelectric power, since the battery 20 is charged, the battery voltageVbat is increased gradually after time t12.

Further, in the direction connection state, the switching loss Pswlossof the DC/DC converter 22 becomes substantially zero. The loss in theDC/DC converter 22 is reduced to the “voltage at the time when the upperarm switching element 112 is placed in the ON state x electrical currentflowing from the secondary side 2S to the primary side 1S of the upperarm switching element 112”. Further, from time t12, at the powergeneration electric power Pfcηg where the net efficiency Netη is at thepeak, power generation control of the FC stack 40 is implemented (stepS71).

During control in transition from around time t11 to time t13, the valueof the system load Psys is stabilized, and during deceleration at theconstant rate from time t13, the system load Psys has a fixed negativevalue (during regeneration of electric power).

SUMMARY OF THE INVENTION

As described above, the fuel cell vehicle 10 according to the embodimentincludes the FC stack 40, the gas supply unit (fuel gas supply unit(regulator 46), oxygen-containing gas supply unit (air pump 60)), theDC/DC converter 22 (voltage regulator unit), the motor 14 (drive motor),the battery 20 (energy storage device), and the ECU 24 (control unit). Afirst gas containing oxygen and a second gas containing hydrogen aresupplied to the FC stack 40, and reactions of these gases are induced bythe catalyst to perform power generation by the FC stack 40. The gassupply unit supplies at least one of the first gas and the second gas tothe FC stack 40. The DC/DC converter 22 regulates the FC voltage Vfc ofthe FC stack 40. The motor 14 is a load driven by the electric power(power generation electric power) Pfc outputted from the FC stack 40.The battery 20 stores electric power regenerated by the motor 14. TheECU 24 controls the FC stack 40, the gas supply unit, the DC/DCconverter 22, the motor 14, and the battery 20.

At the time of regeneration of electric power by the motor 14, the ECU24 places the DC/DC converter 22 in the direct connection state, andstores electric power in the battery 20 while decreasing the oxygenconcentration or the hydrogen concentration by the gas supply unit todecrease the electric power Pfc (FC current Ifc) outputted from the FCstack 40 (step S69).

As described above, at the time of regeneration of electric power by themotor 14, the DC/DC converter 22 is placed in the direct connectionstate under control, the oxygen concentration or the hydrogenconcentration is decreased by the gas supply unit to decrease theelectric power Pfc outputted from the FC stack 40, and electric power isstored in the battery 20 (i.e., the battery 20 is charged with theelectric power). In this manner, unlike the case of Japanese Laid-OpenPatent Publication No. 2006-073506, the contactors CTR1, CTR2 are notopened during power generation of the FC stack 40, and the regenerativeelectric power can be collected highly efficiently. Thus, the durabilityof the contactors CTR1, CTR2 is secured, and there is no concern aboutnoises that would be generated if the contactors CTR1, CTR2 are opened.

At the time of decreasing the electric power (power generation electricpower) Pfc outputted from the FC stack 40, the ECU 24 decreases theelectric power (power generation electric power) Pfc outputted from theFC stack 40 down to the high efficiency range (near Pfcη in FIG. 21)where the net efficiency Netη is at the peak (maximum). Thus, theregenerative electric power Preg can be collected further efficiently.

Further, at the time of regeneration of electric power by the motor 14,when the battery voltage Vbat is equal to or smaller than the lowerlimit voltage v2 of the oxidation reduction progress region R3 of the FCstack 40 (Vbat≦v2×Nfc, step S63: YES), the ECU 24 places the DC/DCconverter 22 in the direct connection state under control, and store theelectric power in the battery 20 under control while decreasing theoxygen concentration or the hydrogen concentration by the gas supplyunit to decrease the electric power Pfc outputted from the FC stack 40.In this manner, degradation of the FC stack 40 is prevented, and theregenerative electric power Preg can be collected highly efficiently.

Further, at the time of placing the DC/DC converter 22 in the directconnection state during regeneration of electric power by the motor 14,the ECU 24 determines whether or not regeneration torque Ttgt exceedsthe threshold value torque Tregth (step S64). If the ECU 24 determinesthat the regeneration torque Ttgt does not exceed the threshold valuetorque Tregth (step S64: NO), then the ECU 24 places the DC/DC converter22 in the direct connection state under control, and stores electricpower in the battery 20 under control while decreasing the oxygenconcentration or hydrogen concentration by the gas supply unit todecrease electric power outputted from the FC stack 40. In this manner,even if the regenerative torque Ttgt is small, the regenerative electricpower Preg can be collected into the battery 20 highly efficiently andreliably.

Further, if the ECU 24 determines that the regeneration torque Ttgtexceeds the threshold value torque Tregth (step S64: YES), the ECU 24does not place the DC/DC converter 22 in the direct connection stateunder control and regulates, by use of the DC/DC converter 22, the FCvoltage Vfc of the FC stack 40 to a value which is equal to or greaterthan the upper limit voltage v3 of the oxidation reduction progressvoltage range (oxidation reduction progress region R3) of the FC stack40, and further the ECU 24 stores electric power in the battery 20 whiledecreasing the oxygen concentration or the hydrogen concentration by thegas supply unit to decrease the electric power Pfc outputted from the FCstack 40. In this manner, even if the regeneration torque Ttgt is large,the large regenerative electric power resulting from this regenerationtorque Ttgt can be collected into the battery 20 highly efficientlywithout degrading the FC stack 40.

Modified Example of the Embodiment

FIG. 24 is a diagram schematically showing a structure of the FC unit 18according to a modified example of the embodiment. In the FC unit 18according to the modified example, in the cathode system 56 a, acirculation valve (cathode circulation valve) 66 is included in additionto the air pump 60, the humidifier 62, and the back pressure valve 64.

In this case, the pipe 66 a, the circulation valve 66, and the pipe 66 bare connected between the pipe 64 b on the output side of the backpressure valve 64 and the pipe 60 a on the air intake side (input side).Thus, some of the exhaust gas (cathode off gas) is supplied as acirculating gas to the pipe 60 a through the pipe 66 a, the circulationvalve 66, and the pipe 66 b. The exhaust gas is mixed with the fresh airfrom the outside of the vehicle, and sucked into the air pump 60.

For example, the circulation valve 66 is a butterfly valve, and theopening degree of the butterfly valve (hereinafter referred to as the“circulation valve opening degree θc” or the “opening degree θc”) iscontrolled by the ECU 24 to regulate the flow rate of the circulatinggas. A flow rate sensor 70 is connected to the pipe 66 b, and the flowrate sensor 70 detects the flow rate Qc [g/s] of the circulating gasflowing toward the pipe 60 a, and outputs the detected flow rate to theECU 24.

As shown a characteristic 167 in FIG. 25, with the increase in thecirculation valve opening degree θc for allowing the exhaust gas to flowthrough the circulation valve 66, the oxygen concentration Co in thecathode channel 74 is decreased.

In this regard, in the modified embodiment, at the time of regenerationof electric power by the motor 14, in the state where the FC voltage Vfcof the FC stack 40 is fixed within a predetermined voltage range(voltage within the stable oxidation region R4 between v3 and v4)outside the oxidation reduction progress voltage range (oxidationreduction progress region R3) of the FC stack 40 by the DC/DC converter22 (e.g., under the mode E control where the FC voltage Vfc is fixed atv3 or Vlmi2 (Vfc=v3 or Vfc=Vlmi2)), or in the state where the DC/DCconverter 22 is placed in the direction connection state (mode Dcontrol, Vfctgt←Vbat), when the target oxygen concentration Cotgt ischanged, only the opening degree θc of the circulation valve 66 ischanged, whereby the FC current Ifc is made variable.

In this manner, in the state where the voltage of the FC stack 40 isfixed to a voltage outside the oxidation reduction progress voltagerange (oxidation reduction progress region R3) of the FC stack 40 by theDC/DC converter 22 or in the direct connection state of the DC/DCconverter 22, the opening degree θc of the circulation valve 66 ischanged to decrease the electric power outputted from the FC stack 40.In this manner, degradation of the FC stack 40 is suppressed (see FIG.8), and in the state where the degradation is suppressed, theregenerative electric power Preg obtained by regeneration is collectedinto the battery 20. Thus, by the decrease in the electric poweroutputted from the FC stack 40, the regenerative electric power Preg canbe collected effectively. Therefore, improvement in the efficiency ofcollecting the regenerative electric power Preg (regenerationefficiency) is achieved while suppressing degradation of the FC stack40.

That is, in the present modified example, at the time of implementingthe regeneration control, unlike the embodiment, without changing therotation number of the air pump 60 and the opening degree of the backpressure valve 64, only the opening degree θc of the circulation valve66 is changed thereby to change the FC current Ifc. Therefore, controlcan be simplified advantageously.

The present invention is not limited to the above described embodimentand the above modified example thereof. The present invention can adoptvarious structures based on the description herein. For example, thefollowing structure may be adopted.

Though the FC system 12 is mounted in the FC vehicle 10, the presentinvention is not limited in this respect. The FC system 12 may bemounted in other objects. For example, the FC system 12 may be used inmovable objects such as ships or airplanes. Alternatively, the FC system12 may be applied to household power systems.

The FC stack 40 and the battery 20 are arranged in parallel, and theDC/DC converter 22 is provided on the near side of the battery 20.However, the present invention is not limited in this respect. Forexample, as shown in FIG. 26, the FC stack 40 and the battery 20 may beprovided in parallel, and a step-up, step-down, or step-up/step-downDC/DC converter 22 may be provided on the near side of the FC stack 40.Alternatively, as shown in FIG. 27, the FC stack 40 and the battery 20may be provided in parallel, and a DC/DC converter 160 may be providedon the near side of the FC stack 40 and the DC/DC converter 22 may beprovided on the near side of the battery 20. Alternatively, as shown inFIG. 28, the FC stack 40 and the battery 20 may be provided in series,and the DC/DC converter 22 may be provided between the battery 20 andthe motor 14.

A unit or a method of adjusting the stoichiometric ratio is performed byadjusting the target oxygen concentration Cotgt. However, the presentinvention is not limited in this respect. Alternatively, target hydrogenconcentration may be adjusted. Further, instead of the targetconcentration, the target flow rate, or both of the target concentrationand the target flow rate may be adjusted.

While the invention has been particularly shown and described withreference to preferred embodiments, it will be understood thatvariations and modifications can be effected thereto by those skilled inthe art without departing from the spirit of the invention as defined bythe appended claims.

1. A fuel cell vehicle comprising: a fuel cell for performing powergeneration by inducing, by catalyst, reaction of a first gas containingoxygen and a second gas containing hydrogen supplied to the fuel cell; agas supply unit for supplying at least one of the first gas and thesecond gas to the fuel cell; a voltage regulator for regulating outputvoltage of the fuel cell; a drive motor as a load driven by electricpower outputted from the fuel cell; an energy storage device for storingelectric power regenerated by the drive motor; and a control unit forcontrolling the fuel cell, the gas supply unit, the voltage regulator,the drive motor, and the energy storage device, wherein at the time ofregeneration of electric power by the drive motor, the control unitplaces the voltage regulator in a direct connection state under control,and stores electric power in the energy storage device while decreasingoxygen concentration or hydrogen concentration by the gas supply unit todecrease electric power outputted from the fuel cell.
 2. The fuel cellvehicle according to claim 1, wherein at the time of decreasing theelectric power outputted from the fuel cell, the control unit decreasesthe electric power outputted from the fuel cell to a high efficiencypower generation range of the fuel cell.
 3. The fuel cell vehicleaccording to claim 1, wherein at the time of regeneration of electricpower by the drive motor, if the voltage of the energy storage device isequal to or smaller than a lower limit voltage value of an oxidationreduction progress voltage range of the fuel cell where oxidationreduction proceeds, the control unit places the voltage regulator in thedirect connection state under control, and stores electric power in theenergy storage device while decreasing the oxygen concentration orhydrogen concentration by the gas supply unit to decrease electric poweroutputted from the fuel cell.
 4. The fuel cell vehicle according toclaim 3, wherein at the time of placing the voltage regulator in thedirect connection state during regeneration of electric power by thedrive motor, the control unit determines whether or not regenerationtorque of the drive motor exceeds a threshold value torque, and if thecontrol unit determines that the regeneration torque does not exceed thethreshold value torque, the control unit places the voltage regulator inthe direct connection state under control, and stores electric power inthe energy storage device while decreasing the oxygen concentration orhydrogen concentration by the gas supply unit to decrease electric poweroutputted from the fuel cell.
 5. The fuel cell vehicle according toclaim 4, wherein if the control unit determines that the regenerationtorque exceeds the threshold value torque, the control unit does notplace the voltage regulator in the direct connection state under controland regulates, by the voltage regulator, the voltage of the fuel cell toa value which is equal to or greater than an upper limit voltage valueof an oxidation reduction progress voltage range of the fuel cell whereoxidation reduction proceeds, and further the control unit storeselectric power in the energy storage device while decreasing the oxygenconcentration or hydrogen concentration by the gas supply unit todecrease electric power outputted from the fuel cell.